Planographic printing plates having coatings containing insoluble xanthates

ABSTRACT

Printing plates comprising a base having a planographic coating thereon of a water-insoluble polyvalent metal xanthate of a hydroxy-containing polymer containing a pigment. This application is a continuation in part of my prior application, Ser. No. 491,869, filed Sept. 30, 1965.

United States Patent Inventor Appl. No.

Filed Patented Assignee Glenn I1. Perkins Mexico, Maine Dec. 4, 1968 No Drawing Continuation-impart of Ser. No. 491,869, Sept. 30, 1965.

Feb. 2, 197 1 Ethyl Corporation a corporation .of Virginia PLANOGRAPIIIC PRINTING PLATES HAVING COATINGS CONTAINING INSOLUBLE XANTIIATES 10 Claims, No Drawings US. Cl

[51] 1nt.Cl B41n 1/00, B4ln 3/00 [50] Field ofSearch 101/460, 461,462; 260/2335; 1 17/621, I56

[56] References Cited UNITED STATES PATENTS 3,160,552 12/1964 Russell et a1 260/233.5X 3,429,735 2/1969 Bridgeford 117/62.1X

Primary Examiner-David Klein Attorney-Pennie, Edmonds, Morton, Taylor and Adams ABSTRACT: Printing plates comprising a base having a planographic coating thereon of a water-insoluble polyvalent metal xanthate of a hydroxy-containing polymer containing a pigment.

PLANOGRAPHIC PRINTING PLATES HAVING COATINGS CONTAINING INSOLUBLE XANTHATES This application is a continuation in part of my prior application. Ser. No. 491,869. filed Sept. 30, l965.

This invention relates to coating compositions useful in making planographic printing plates. More particularly, this invention relates to planographic-printing plates formed by coating a suitable base with a coating composition comprising clay and a xanthate of a hydroxy-containing polymer. The coating composition is then dried and the xanthate is insolubilized by means of a suitable polyvalent metal salt.

Planographic-printing plates are made by applying to a base web or sheet a coating having, among other things, desirable hydrophilic-oleophilic properties, whereby a greasy composi' tion may be retained, to form an ink receptive image, and an aqueous etch or fountain solution may be retained by the remainder of the plate to form an ink repellent surface. Such plates may be mounted on a roller of an offset printing press, for example, and caused to pass successively in contact with a fountain solution and a printing ink. Ideally, the ink is totally rejected by the plate, except for the imaged areas thereof to which the ink adheres. The inked image is then transferred to a blanket, for image reversal, and applied to the sheet to be printed.

The making of superior planographic-printing plates requires delicate balancing of a variety of coating characteristics, some of which tend to conflict with others. Byway of example, the coating must accept and retain a sharp, greasy image, while at the same time accepting and retaining the aqueous fountain solution. The fountain solution must be retained in such manner as to avoid undermining of the imagein prolonged use. The coating must permit of the making of erasures, when necessary, without causing halos or other undesired images to appear. The image must remain sharp throughout prolonged use, and not tone." Stop-go characteristics, that is, the ability to print satisfactory first copolymers known in the art. The flour or wheat flour referred to above is essentially a mixture of various compounds containing hydroxyl groups such as starch, sugar, hemicellulose,

pies after a period of machine shutdown, must also be good. I

These and other characteristics must be properly balanced to provide a superior plate, due regard being had for ease of manufacture and overall cost.

The present invention provides an improved planographicprinting plate, which is superior in the characteristics abovementioned, among others, which is capable of economical manufacture, and which may be varied easily to accommodate a variety of specific uses. The use of the xanthates of hydroxycontaining polymers not only produces the superior resuits described above, but also provides an improved process wherein such results are obtained by readily insolubilizing the xanthates with polyvalent metal salts.

The present invention resides in the discovery that suitable planographic-printing plates may be prepared without the use of the usual hydrophilic colloid adhesives when a xanthate of a hydroxy-containing polymer, such as a starch xanthate, is employed together with a pigment, such as clay, in the plano graphic coating. The xanthate is insolubilized by washing the dried coating with a polyvalent metal salt solution.

Broadly, the improved planographic-printing plates of this invention are prepared by forming an aqueous coating composition comprising an inorganic pigment, such as clay, and a xanthate of a hydroxy-containing polymer, said xanthate being used as the adhesive in the coating composition. This aqueous coating composition is then applied in a conventional manner to the surface desired, usually a wet strength paper web, and the coating is at least substantially dried, advantageously at a pH of about 9 to ll. The dried coating is thensalted with a polyvalent metal salt solution to form an insoluble pianographic coating.

According to this invention, the xanthates of various hydroxy-containing polymers can be used as adhesives in the coating compositions for the planographic surfaces. The xanthates which may be used are xanthates of the various starches and flours, such as potassium amylose xanthate, potassium starch xanthate, potassium wheat flour xanthate and other suitable xanthates of hydroxy-containing natural gluten, and so forth, and each of which form derivatives with carbon disulfidc in the xanthation reaction. A hydroxy-containing natural polymer is a natural polymer-containing hydroxy groups, as well known by those skilled in the art. Further examples of such polymers are dextran, dextrin, guar gum, corn hull gum, locust bean gum, and the like. Synthetic polymers containing hydroxyl groups can also be advantageously employed to form the xanthates used as adhesives in forming planographic-printing surfaces. Numerous synthetic hydroxy containing synthetic polymers, well known to those skilled in the art, can be used including polyvinyl alcohol, 2-hydroxy-ethyl-methacrylate, 2-hydroxy-propylmethacrylate, and so forth.

The various xanthates may be prepared by treating the polymer, such as ordinary starch or flour, at room temperature with potassium hydroxide and carbon disulfide to form a potassium starch or flour xanthate. The potassium xanthate is purified by precipitation of the potassium salt in cold methanol, and washing it with several portions of cold methanol followed by washing with ether. The dried potassium xanthate is then dissolved in distilled water to form a solution that is used in preparing the coating composition. The viscosity of the xanthate solution may be varied quite widely depending on the desired viscosity of the coating compositions for proper application to the base stock. Viscosities of from between 300 and 800 centipoises, Brookfield, have been advantageously used.

Various known coating or filler inorganic pigments can be used according to this invention. Clays of small particle size are preferred since they seem to produce better results. Some examples of clays which can be used include various types of kaolin or china clays, Lustra Clay, etc. Lustra Clay marketed by the Southern Clay Company has been found to be particularly advantageous. Other pigments may be used alone or in combination with the clay if desired, such as calcium carbonate, barium sulfate, silica, titanium dioxide, calcium sulfate, magnesium silicate, etc. A pigment comprising Lustra Clay along or in combination with silica is preferred. When silica and Lustra Clay are used together, the ratio of clay to silica may vary widely from about 5:3 to about 111 depending upon the properties desired in the finished plate, the particular xanthate adhesive employed, the presence of other inert materials or pigments in minor proportions, etc.

The colloidal silica which can be used to form the pigment mixture of this invention advantageously contains silica particles of very small size. The main average size of the silica particles should be within a range of from about 7 millimicrons to about 30 millimicrons. The colloidal silicas of this size generally result in the production of planographic-printing plates having improved properties. Larger sized colloidal silica particles can be used however. Although colloidal silicas-containing silica particles of less than 7 millimicrons have not been commercially available, it is believed that the coating compositions of this invention may be advantageously prepared from colloidal silicas-containing silica particles of a size even less than 7 millimicrons.

The mostadvantageous colloidal silicas are those which contain silica particles ranging in size from about 7 to 17 millimicrons. Such colloidal silicas are marked by [5.1. duPont de Nemours and Company under the trade names Ludox S.M., Ludox LS, and Ludox H.S., and also by the Monsanto Chemical Company under the trade name Syton 200. The types of colloidal silicas and methods of making them are described in detail in US. Pats. No. 2,244,325, issued June 3, 1941 to Bird, US. Pat. No. 2,574,902, issued Nov. 12, 1951 to Bechtold and Snyder, and us. rat. No. 2,597,872, issued May 27, I952 to ller. Such colloidal silicas are generally marketed as a 30 percent SiO, solution, but as described in the above patents, the colloidal silica can be prepared containing a higher or lower percent silica.

Polyvalent metal salt solutions are used to insolubilizc the coating compositions. The dried coating compositions are salted with about 8 percent to 12 percent solution of the polyvalent metal salt. The polyvalent metal salts that may be used include zinc acetate, copper sulfate, lead acetate. and ferric chloride. as well as salts of other divalent and trivalent metals, e.g., calcium, manganese, cerium, and lanthanum.

The coating compositions according to this invention are advantageously prepared by adding the starch or flour xanthate solution to the clay slip while stirring. After thorough mixing add the butyl alcohol. The mixing should be done at ambient temperatures, and the viscosity of the mixture is advantageously adjusted from about 100 to 500 centipoises, Brookfield.

The coating composition may be applied to a base web or sheet by conventional apparatus, such as air knives or roll coaters. The amount of coating may vary from about 2 to pounds per ream of 3300 sq. feet, with 6 to 8 pounds being most satisfactory. After coating, the stock is dried at a suitable temperature. Temperatures as low as 140' F. and as high as 350 F. have been used, although even lower or higher temperatures may also be used. The drying time may advantageously range from about 2-20 seconds or more, as may be readily determined by one skilled in the art. The coating composition is insolubilized when washed with a polyvalent metal salt solution, such as zinc acetate, to produce a practically water impervious film which possesses excellent properties of accepting and holding both water and greasy ink.

The ratio of the pigment to the xanthate may vary considerably depending upon various factors such as the particular pigment employed. The pigment/xanthate ratio may vary from 10:1 to 3:1, with a preferred range between 4:] and 6:1.

The following examples illustrate the procedure of producing the coating compositions and planographic-printing plates of this invention in more detail.

EXAMPLE 1 Grams Potassium starch xanthate (8%)- 62. 5 62. 5 K08 clay (60% suspension)..... 25. 0 33. 0 Ludox H.S 33. 0 Butyl alcohol 5.0 5. 0

These compositions were coated onto separate base sheets by the air knife method, and allowed to dry at room temperature. The coated sheets were then washed with a solution of 12 percent zinc acetate and dried. The sheets were run on the press and produced good, clean copies, even after prolonged use and possessed good stop-go characteristics.

EXAMPLE 2 The procedure of Example 1 was followed, except: (a) ordinary wheat flour was treated instead of the corn starch and (b) the coating composition prepared and applied to the base sheet was as follows:

Grams Potassium wheat flour xanthate (8%) 100 Lustra clay (60% suspension) 33 Butyl alcohol 3 The sheets were run and produced copies similar to those in Example 1.

EXAMPLE 3 The procedure of Example I was followed except that the dried coating compositions which adhered to the base papers were washed with 10 percent alum and I0 percent ferric chloride solutions. The sheets were run and produced copies similar to those in Example 1.

EXAMPLE 4 The procedure of Example 1 was followed, except: (a) amylose was treated instead of corn starch and (b) the coating composition prepared and applied to the base sheet was as follows:

Grams Potassium amylose xanthate (10%) KCS clay (60% suspension) 33 Butyl alcohol 2 xanthate is a zinc starch xanthate.

5. The planographic-printing plate of claim 1 wherein the polyvalent metal is iron.

6. The method of making a planographic-printing plate which comprises applying a planographic-printing surface to a base by coating said base with an aqueous coating composition comprising an inorganic pigment and a xanthate of a hydroxy-containing polymer, drying the coating composition then salting said dried coating with a solution of a polyvalent metal salt so as to form a water-insoluble polyvalent metal xanthate of the hydroxy-containing polymer and placing a hydrophobic image on said insolubilized coating.

7. The method of claim 6 in which the pigment contains at least about 50 percent by weight of clay.

8. The method of claim 7 in which the hydroxy-containing polymer is a natural polymer.

9. The method of claim 8 in which the xanthate is a potassium flour xanthate.

10. The method of claim 6 in which the polyvalent metal is zinc. 

2. The planographic-printing plate of claim 1 in which the pigment contains at least about 50 percent by weight of clay.
 3. The planographic-printing plate of claim 1 in which the hydroxy polymer is a natural polymer.
 4. The planographic-printing plate of claim 3 wherein the xanthate is a zinc starch xanthate.
 5. The planographic-printing plate of claim 1 wherein the polyvalent metal is iron.
 6. The method of making a planographic-printing plate which comprises applying a planographic-printing surface to a base by coating said base with an aqueous coating composition comprising an inorganic pigment and a xanthate of a hydroxy-containing polymer, drying the coating composition then salting said dried coating with a solution of a polyvalent metal salt so as to form a water-insoluble polyvalent metal xanthate of the hydroxy-containing polymer and placing a hydrophobic image on said insolubilized coating.
 7. The method of claim 6 in which the pigment contains at least about 50 percent by weight of clay.
 8. The method of claim 7 in which the hydroxy-containing polymer is a natural polymer.
 9. The method of claim 8 in which the xanthate is a potassium flour xanthate.
 10. The method of claim 6 in which the polyvalent metal is zinc. 